A hydraulic vehicle brake system with an anti-skid system and a control device is known from U.S. Pat. No. 4,809,183. Furthermore, a hydraulic pressure source and at least one evaluating circuit are associated with the anti-skid system for detecting an oversteering tendency, an understeering tendency and the start of the danger of skidding, and for generating control signals for the anti-skid system, so that this system automatically brakes the vehicle with the use of the hydraulic pressure source, and from case to case the wheels of one wheel axle or of all wheel axles are braked. The automatic braking stabilizes the vehicle, thus counteracting an oversteering tendency, an understeering tendency, or a danger of skidding, or skidding that is already in progress. The pressure source is not shown. To obtain the control signals, king pin inclinations are measured, for example, and compared with preselected threshold values. The rotating velocity of the vehicle around the vertical axis can also be observed by means of a gyroscope, for example. Alternatively, transverse accelerations can be measured by means of acceleration sensors associated with the axles and evaluated.
German Patent Disclosure DE 41 09 025 A1 discloses a hydraulic vehicle brake system with advantageous, closed brake circuits and an anti-skid system that operates in accordance with the so-called recalculating pump principle, and in which a combination of a controllable 2/2-way valve and a cylinder with a separating piston and a restoring spring for the separating piston is installed for each brake circuit, between this anti-skid system and a main brake cylinder, to control the cornering of a vehicle; a common auxiliary pump and a 3/2-way valve disposed downstream of it are also provided for both brake circuits for controlled charging of the separating piston and to control the 2/2-way valves, which are hydraulically controllable by means of control inputs for closing the brake lines between the cylinders and the main brake cylinder. Pressures generated in the cylinders after the main brake lines have been blocked are modulated by means of the anti-skid system and supplied individually to the wheel brakes of the four wheels. The anti-skid system can also individually reduce the pressures supplied to the wheel brakes. A control device that controls both the 3/2-way valve and the anti-skid system to improve the cornering of the vehicle equipped in this manner can also activate a driving motor for the auxiliary pump. The disadvantage is that, because of the hydraulic connection of the cylinders with modulation valve arrangements of the anti-skid system, the auxiliary pump must deliver a pressure at the level of the highest brake pressure to be expected during cornering. Correspondingly, the auxiliary pump must be embodied to be powerful; a required driving motor is heavy and stresses an electrical system of the vehicle in a disadvantageous manner. The heavy driving motor and the heavy auxiliary pump are expensive. A further disadvantage is that, because of the high pressure generated, disturbing and possibly very aggravating noises occur during pump operation. A reservoir that can be charged by the auxiliary pump is also expensive, as is the electrically controllable 3/2-way valve that controls the charging of the separating piston and the hydraulic control inputs of the 2/2-way valves. Moreover, because of the embodiment of the cylinder and its separating piston, the respective 2/2-way valve must be combined with a check valve function, so that during automatic brake operation the transition into normal brake operation can immediately take place solely through operation of the brake pedal. Therefore, in the disclosed exemplary embodiment the symbol of a check valve is allocated to the second position of the 2/2-way valves.
It is known from U.S. Pat. No. 4,412,701 to further embody a hydraulic vehicle brake system with a main brake cylinder and wheel brake cylinders for automatic distance control from a preceding vehicle, and/or to limit wheel slip. For this purpose a cylinder with a separating piston embodied as a stepped piston and displaceably disposed therein is installed for each brake circuit, between the main brake cylinder and the wheel brake cylinders. An auxiliary pressure source is associated with the cylinders. The separating piston has a lip sealing ring in the region of its large diameter, and there defines a primary chamber that can be charged by pressure from the auxiliary pressure source. In the region of the opposite end of the separating piston, which has a smaller diameter, the separating piston has a further lip sealing ring that defines a secondary chamber within the cylinder. A valve opening in the wall of the cylinder is associated with this lip sealing ring. When the separating piston is displaced, this lip sealing ring travels across the valve opening and, in this way, forms a 2/2-way valve with the opening. The valve opening is connected to the main brake cylinder. The secondary chamber: communicates with wheel brake cylinders. In this respect, braking is possible on the one hand by means of pressure from the main brake cylinder by operating a brake pedal and, on the other hand, automatically by means of introducing auxiliary pressure from the auxiliary pressure source into the primary chamber, wherein the auxiliary pressure displaces the separating piston, closes the 2/2-way valve and generates the required brake pressure in the secondary chamber. A third lip sealing ring whose sealing lip likewise points toward the secondary chamber is disposed between the two lip sealing rings. Between the third lip sealing ring and the lip sealing ring on the side of the primary chamber, the cylinder has a pressure equalization opening open to the outside. It is disadvantageous that leakage caused by wear or damage to the lip sealing ring associated with the valve hole cannot be detected during normal braking operations; rather, it is first detected when automatic braking is intended to be initiated, and either no braking effect or a braking effect of inadequate duration results. When this leakage is present, the middle lip sealing ring should assure normal braking with the operation of the main brake cylinder. Whether the middle lip sealing ring itself is leaky cannot be detected during automatic braking, and can only be detected by chance with normal braking by means of the main brake cylinder when the main brake cylinder and the lip sealing ring associated with the valve hole are leaky. If both seals are leaking, the additional disadvantage arises that the pressure medium flows around both lip sealing rings, and the pressure medium escapes into the environment through the pressure equalization opening. By means of this, the vehicle brake system is emptied, so that a braking effect can no longer be generated.
German Patent Application P 42 32 311.8 (DE 42 32 311 A1) proposes to provide a hydraulic vehicle brake system designed for automatic braking for the purpose of improving the driving performance of a vehicle (driving dynamics control) and/or traction control with a cylinder with a separating piston displaceably disposed therein, and an auxiliary pump for automatically charging the separating piston whose pressure is significantly lower than the full brake pressure generated in the main brake cylinder, between a main brake cylinder connection and an anti-skid system, that operates in accordance with the so-called recirculating pump principle. To assure operational reliability and monitoring capability with regard to operational reliability, the separating piston has two piston sealing rings. Moreover, the piston, connected to a conduit leading out between the piston rings and communicating with the main brake cylinder, includes a 2/2-way valve embodied as a seat valve with a ball embodied as a sealing element that is closed when the piston is displaced by means of auxiliary pressure. A 2/2-way valve in seat design, built into the separating piston, is expensive. A further exemplary embodiment proposed by this German patent application P 42 32 311.8 (DE 42 32 311 A1), with a cylinder and a 2/2-way valve, additionally includes a floating piston that likewise has two piston sealing rings for reasons of operational reliability. This exemplary embodiment is even more expensive than the first exemplary embodiment mentioned. In both exemplary embodiments it is provided that the cylinders can be connected via electrically operable 2/2-way valves to intakes of the recirculating pumps for the purpose of automatic braking, so that the recirculating pumps can exert high pressure for braking on the pressure medium originating from the cylinders.